1. Field of the Invention
The invention generally relates to scanning antennas and, in particular, to microwave landing systems using a signal format which requires multiple antenna functions to provide the signals over wide coverage sectors.
2. Description of the Prior Art
Antenna techniques are known which use the phased array scanning beam antenna of a microwave landing system (MLS) to broaden its radiation pattern to satisfy the data antenna requirements. Such techniques generally employ phase spoiling of the phased array aperture. The fundamental limitation of this technique is that it cannot provide the out-of-coverage indications (OCI) signals and the 360.degree. data signals without employing a single thread multiple port rf switch. This technique is deficient because it is subject to single point failure within the phased array which can cause substantial radiation pattern minima when used in the data antenna mode (low gain - broad pattern). These minima are very difficult, if not practically impossible, to monitor and detect. In addition, single point system failures also exist and can create significant safety risks in certain operational scenarios.
The MLS signal format requires multiple antenna functions to provide the signals over wide coverage sectors. The format also provides for signal transmission outside the normal coverage volume, e.g., out-of-coverage indication signals (OCI). Inherent growth capabilities in the system such as 360 data link coverage, also require additional antennas in many practical applications. Because of the multiple antennas required for MLS, an antenna switch is used to connect a transmitter sequentially in time to each antenna port. Although redundant transmitters and control electronics can be enacted on line to provide signal continuity in the event of a failure, the switching components (rf and logic) are a limiting factor in supporting the requirements for signal continuity in high reliability applications.